doc: enable line number continuation in the pH example

[franckgaga]

Also removed the sentence about convergence of the estimated parameter. This is no longer an issue with the bugfix of #459.

Preview : https://juliacontrol.github.io/ModelPredictiveControl.jl/previews/PR460

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.21%. Comparing base (bc3be8e) to head (b6d5ca1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
+ Coverage   97.16%   97.21%   +0.05%     
==========================================
  Files          28       28              
  Lines        6867     6867              
==========================================
+ Hits         6672     6676       +4     
+ Misses        195      191       -4     

Flag	Coverage Δ
unittests	97.21% <ø> (+0.05%)	⬆️

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